This invention relates to an improvement of the phase characteristics of a bandpass filter which is used to select a signal component from a wideband frequency spectrum.
In general, the phase difference between the input and output of a bandpass filter for an input frequency within the filter passband is not constant but instead is frequency dependent. This is illustrated in FIG. 1 of the drawings.
In FIG. 1, the X axis represents frequency, while the Y axis represents attenuation and phase. The solid line shows the attenuation characteristic of a typical bandpass filter and the dotted line shows its phase characteristic. In addition, f.sub.-1 and f.sub.1 are the lower and upper filter cut-off frequencies, and f.sub.o defines the filter's center frequency.
As shown in FIG. 1, as the input frequency becomes far from the center frequency f.sub.o, a phase deviation between the input and output signals increases. The increasing phase deviation becomes significant as the bandwidth of a bandpass filter is reduced. Therefore, when it is necessary to maintain a constant phase relationship between the input and output signals in a circuit where the signal component is selected by means of a narrow bandwidth bandpass filter, the input frequency and the center frequency of a bandpass filter should always coincide.
A bandpass filter having such requirements is widely used for a data transmission system where the synchronization signal or carrier is selected at the correct phase (e.g.--for data transmission by means of a PSK modulation system).
A carrier recovery circuit for 4-phase PSK system is shown in FIG. 2 as an example.
As illustrated in FIG. 2, the 4-phase PSK signal is quadrupled by the frequency multiplier 21. Then, the noise component is reduced by the narrow band bandpass filter 22. Thereafter, the resultant signal is divided by 4 by the frequency divider 23. Thus, the carrier signal is extracted.
In the above-noted method, it is necessary to substantially eliminate the noise component included in the output of multiplier 21 and on the other hand it is also necessary to keep the phase deviation small because such a phase deviation between the input and output signals of the bandpass filter 22 will deteriorate the system performance if the input signal frequency changes.
For this reason, a tracking type filter is usually employed. In the tracking type filter shown in FIG. 3, the phase difference between the input and output signals of a bandpass filter 32 is detected by a phase detector 34. A voltage corresponding to the detected phase difference is transmitted from the phase detector 34 to a local oscillator 36 via a loop filter 35, thereby causing the oscillation frequency of the local oscillator 36 to be changed so that the center frequency f.sub.o of the bandpass filter 32 and the frequency f of the input signal to the bandpass filter 32 coincide.
However, such a tracking type filter has a problem in that the tracking of the transitionary input signal is delayed because of the feedback control.